The invention concerns a flat sheet injection head extruder.
In apparatus of this kind the extruder plasticizes a compound and conveys the plasticized material into the flat sheet injection head, by which the same is formed into a wide strip. Directly adjoining the exit of the flat sheet injection head there can be disposed for example the gap or mouth of a two roll calender wherein the plastic material is rolled into a continuous web of constant thickness and width.
After completion of the process, any material remaining in the flat sheet injection head must be removed since, if it is permitted to stay in the head, it solidifies and consequently entails the risk of cracking the injection head, calender rolls and even the extruder when the installation is started up again. Furthermore, the flat sheet injection head must also be cleaned with a change of mixture (e.g. when changing color or composition of the mixture) since thorough rinsing of the flat sheet injection head takes too much time and also results in a considerable loss of material.
To facilitate removal of material from the fish tail shaped channel of flat sheet injection heads the latter are generally divided in a horizontal plane and, in operation, are screwed together or hydraulically clamped together.
Flat sheet injection heads specifically used in roller head plants, as described, for example in German Patent Specification Nos. 1,704,490 and 2,028,065, are designed so that one half of the head is firmly joined with the calender and the second half is firmly joined with the extruder. In operating position, the two halves are clamped together hydraulically and held in this position by means of devices mounted at the left and right of the injection head on the two calendar stands. If the flat sheet injection head is to be cleaned, the clamping is released and the entire extruder together with the half of the head fastened thereon is shifted on a carriage or a roller bed hydraulically or mechanically in a horizontal direction for such a distance that the two halves of the head are completely separated from each other. The material left behind in the injection head, which in most cases adheres to the half attached to the extruder, can now be removed without much effort. In this way, the installation is ready for processing new material in a minimum of time.
It has been found, however, that with larger widths of sheets and thus also wider flat sheet injection heads the two halves of the head with this kind of clamping will bend due to pressure of the mass to such an extent that material will escape particularly at the center of the parting line.
These wider injection heads must consequently be made to resist greater bending forces. This, in turn, results in greater weights of the heads which consequently react sluggisly to temperature regulating impulses to provide the required heating or cooling. In spite of the increased bending resistance or stiffness it is necessary in most cases to use additional joining means such as screws, clamps or the like between the center and edges of the injection head. Such additional joining means not only increases the cost, but loosening of the same requires a considerable expenditure of time.